Particle coating process



May 31, 1966 D. E. WURSTER PARTICLE COATING PROCESS Filed Jan. 13, 1964 INVENTOR.

DALE E WURSTER A TTOR/VEY United States Patent 3,253,944 PARTICLE COATING PROCESS Dale E. Wurster, Madison, Wis., assignor to Wisconsin Alumni Research Foundation, Madison, Wis, a corporation of Wisconsin Filed Jan. 13, 1964, Ser. No. 337,481 1 Claim. (Cl. 117-400) The present application is a continuati-on-in-part of copending application Serial No. 861,063, filed December 21, 1959.

The present invention relates to the coating art and more specifically to improvements in the air suspension coating process. This process and illustrative apparatus for carrying out the process are described in my prior Patents No. 2,648,609 and No. 3,089,824. As shown in these disclosures, the particles to be coated, e.g., tablets and the like, are suspended in a coating tower in a moving gas stream, e.g., an air stream, and the coating composition in atomized or like form is introduced into the gas stream prior to its contact with the suspended particles to be coated.

In practice it has been found that improved uniform coating can be obtained by the controlled cyclic flow in the coating tower of the particles being coated. It has also been found that a definite cyclic pattern can be established with aeration of the complete tower to provide a suspended bed of particles in which all particles are suspended in an upwardly flowing gas stream, by varying the velocity of the gas in different parts of the tower. A preferred way of carrying this out is by producing an upward movement of particles in the part or side of the tower in which the coating composition is introduced by maintaining the gas in this part of the tower, where the application of the coating to the particles takes place, at a velocity sufficient to lift the particles upwardly, surrounded'by a cushion of air, and substantially out of contact with each other until the coating is substantially dry. At this point the particles then pass to the top portion of the other part or side of the tower where the velocity of the gas is maintained insufficient to keep the particles from moving downwardly but adequate to maintain them in substantially weightless contact with each other as they move in a streamline fashion downwardly and return to the high velocity part of the tower.

This type of operation allows a large area of the particles to be coated in each pass through the coating section of the tower with a liquid coating of suflicient thickness to give a smooth surface. Agglomeration is also prevented by maintaining the particles out of contact with each other until the coating is dry. In addition, possible smearing or attrition of the coating is prevented by maintaining the particles in substantially weightless contact with each other during the return cycle.

In practice is has been found that if the coating is applied to only a small part of the tablet or like particle, e.g., a few droplets at one time, the tablets tend to develop rough surfaces and may even develop what is known in the art as ears. I have discovered that this can be avoided if the coating is applied during each pass or cycle to a large or substantial part of the tablet, e.g., one side of the tablet, at one time.

Fluidized beds in general are characterized by randomness of particle motion. It has been found that such random motion is undesirable particularly when coating tablets or larger particles. Therefore velocity differences which set cyclic flow patterns as herein described are fundamental to the rapid application and development of smooth elegant surfaces. If particles are caused to move in and out of the coating zone randomly, the coating can be applied only at a slow rate. Particle contact must be delayed until suflicient drying has occurred to' prevent sticking. With random action the drying time is indeterminate. Conversely when a cyclic flow pattern is established by controlled velocity differences as described below, without randomness of particle motion characteristic of fluidized beds, relatively long periods of time can exist between collisions or contacts between particles, and more drying can be accomplished.

By the procedures described the particles are given upward velocity just as they are returned to the coating zone. This velocity is sufficient to transport the particles to the top of the bed by which time the gas velocity has decreased and become uniform by diffusion, and the particles drop to the surface of the remaining portion of the bed. When velocity differences are maintained as described, the tablets are also supported by air to an extent that they are practically weightless during that portion of the cycle by which they are returned to the coating zone.

During the period of vertical upward travel the tablets or other particles are also substantially separated from each other by films of the air or gas used for supporting the bed. In this manner a specific time is provided for drying or removal of the coating solvent under ideal conditions, i.e., no contact of particles and maximum gas velocity, and fast coating rates result. Maximum coating rates are important, for only under those conditions can a substantial part of the tablet surface be coated per pass or cycle and thus provide a smooth rather than rough or sandy surface. 7

For a more complete understanding of the practical application of the principles of my invention reference is made to the appended drawings which show an apparatus suitable for carrying out the process of the invention. In the drawings:

FIGURE 1 is a fragmentary vertical cross-section of the lower portion of the apparatus;

FIGURE 2 is a cross-sectional view taken at line 22 of FIGURE 1; and

FIGURE 3 is an alternative arrangement of the grid shown in FIGURE 2.

Referring more particularly to FIGURE 1, the apparatus basically includes a vertically disposed tower 10, an air supply system and a coating material feed system. Tower 10 is a vertically elongated hollow shell, con structed or rigid, hard material such as steel or aluminum, or a transparent synthetic resin (e.g., Lucite), or similar material or combinations of these. At its lower end tower 10 includes a frustoconical lower end or throat section 12, reducing somewhat the diameter of the opening into tower 10 at its lower end.

Depending from throat section 12, tower 10 is provided with an elongated nozzle section 14 which com municates at its upper end with the opening into section 12. The lower end of nozzle section 14 terminates in a generally curved cylindrical conduit section 16 into which the air supply system is connected. A grid 18 in the form of a pair of screens 20 and 22 is mounted in the upper end of nozzle section 14 extending completely across the interior of such section. The screens may be mounted on a frame portion 24 which is connected across the lower end of section 12. As shown in the drawing, the air stream entering the bottom of tower 10 is substantially coextensive with the cross-sectional area of the suspended bed of particles in the tower. This is essential for proper aeration of the complete tower and specifically of the downwardly moving bed, which moves downwardly countercurrent to the upwardly flowing, low velocity air stream, as well as for proper aeration of the upwardly moving bed, which moves upwardly concurrent with the upwardly flowing, high velocity air stream.

Alternatively, as shown in FIGURE 3, the grid 18 may be fabricated without the diametric support 24 by allowing the coarse mesh screen 20 to cover the entire opening from section 14 to the frustoconical section 12, and fastening fine mesh screen'22 to the coarse mesh screen Fine mesh screen 22' does not cover the entire opening but is partly cut away. Screen 22' may be a semicircle covering one side of screen 20' to give the same effect as the embodiment of FIGURE 2, or a smaller or different shape segment may be cut out.

The upper portion of tower (not shown) may be further provided with a disengaging means of known design to facilitate disengagement of entrained fines, and also with an exhaust connection and a stack or fine or solvent recovery'system. See Patent No. 3,089,824, supra. The air supply system is conventional in nature and includes suitable provision for controlling humidity of and for heating air which is blown into the lower end .of section 16 through an air conduit 26 which is connected directly to section 16.

The coating feed material system which is more completely described in Patent No. 3,089,824, basically is arranged to introduce the coating materials, which are dissolved or dispersed or dissolved and dispersed in a volatile fluid, under suitable pressure through an inlet connection 28 of an atomizer 30 extending into the lower end of nozzle section 14. The material feed system further includes a second air inlet connection 32 leading into atomizer 30 for admitting compressed air to assist in atomizing the coating materials. It will be noted that compressed air inlet 32 and coating material inlet 28 enter section 14 to one side of that section (left side in the drawings) and the fluids are directed at an angle toward each other at the outlet end to form atomizer 30.

The grid 18 which is located at the lower end of throat section 12 consists as described of two preferably semicircular screens and 22 which are adjacent on a com- 'mon support 24. Screen 20 has a relatively coarse mesh, e.g., 10 mesh, and is positioned directly above nozzle on the left in the drawings. Screen 22 has a relatively fine mesh, preferably consisting of several layered sections of fine mesh, e.g., 200 mesh, screen material and is positioned to the right in the drawings. Fine mesh screen 22 offers considerably more resistance to air flow, so that the greater part of the air introduced into supply conduit 26 will be diverted to the coarse mesh side (left in the drawings) and pass into the tower 10 with a greater velocity on the left or coating side of the tower than on the right side of the tower.

In operation a charge of particles to be coated, such as tablets is admitted to tower 10. This charge is retained on porous grid 18 having areas of differing porosity. Gas, e.g., warm air, is introduced into the tower 10 through inlet connection 26. The velocity of air entering tower 10 through inlet connection 26 is adjusted such that the charge of tablets to be coated is suspended in throat section 12. It has been noted that by reason of the employment of a coarse screen 20 on one side of the inlet to section 12 and a fine screen 22 on the other side a greater quantity of air is permitted to enter section 12 on one side (left as illustrated) than on the other. As a consequence the bed is unequally supported by the air stream passing upwardly through nozzle section 14. The effect of this is to produce a general 'moving flow of tablets T upwardly on the left side of the bed and downwardly on the right side. Moreover, as tablets T commence to flow upwardly immediately above screen 20 in the relatively higher velocity air stream they are carried apart substantially out of contact with one another and move at a relatively rapid rate, for example, approximately 2' per second in the case of tablets having a diameter of approximately /8. As tablet-s Treach the upper part of tower 16, they substantially stop moving upwardly because of the reduced velocity of the air stream caused by the resistance of the tablets and diffusion of the stream. Tablets T then spill over into the still lower velocity air stream on the right side of bed B, where they are permitted to fall gently and in substantially weightless contact, supported by the upwardly flowing air stream issuing through fine mesh screen 22, until they reach a point in section 12 (or above grid 18) where the air stream has velocity sufficient to support them. At this point the particles move to the left from directly over screen 22 to directly over screen 20 by the continuing flow of tablets descending from above. Thus tablets are introduced again into the higher velocity air stream issuing through screen 20 and are again carried up to repeat the cycle.

It will be apparent from the previous description that by reason of the relatively higher rate of tablet movement in the high velocity air stream side of tower 10 the bed is substantially less dense in that side than where it moves downwardly in the lower velocity air stream. Aspointed out above, the maintenance of a number of tablets substantially out of contact with each other is a desideratum in order to coat-the tablets in accordance with the process of this invention. Atomizer 30 is positioned to introduce all of the coating material into the air stream which has the relatively higher velocity so as to provide for the coating of only the tablets moving upwardly and after they have achieved substantially their maximum upward velocity in the high velocity air stream. Once the bed of tablets has been established with tablets T flowing upwardly through one portion of the bed and downwardly through a second portion of the bed B, the coating materials are introduced through inlet connection 28 and atomized in nozzle 30 by compressed air introduced through inlet connection 32 which impinges against the coating materials. The atomized coating materials as noted above are carried by the relatively higher velocity air stream passing through screen 20 directly into contact with the well separated tablets T in the upwardly rising portion of the bed.

By reason of the spacing of tablets T in this portion of the bed as they rise upwardly in the higher velocity air stream, the coating materials generally tend to coat at least a half of each tablet T contacted by the coating materials thereby avoiding the possibility of the growth of rough surfaces on the tablets. As the tablets T which have been partially coated by the coating materials rise upwardly in the bed, the coating material on them is dried by the warm gas or air stream supporting them such that by the time'the partially coated tablets T reach the upper part of tower 10 and spill over into the downwardly flowing portion of the bed B supported by the lower velocity air stream issuing through screen 22, the coating materials on such tablets have substantially dried, and the contact between tablets.which occurs in the downwardly flowing, more dense portion of bed B does not. cause the tablets to agglomerate. As the cyclic movement of each tablet in the bed continues the tablet presents itself above or in front of nozzle 30 many times and in many different positions, permitting a coating to be built uniformly about the tablet. When the desired thickness of coating has been built up on the bed as a whole, which occurs at a fairly uniform rate for all the tablets in the bed, the admission of coating materials into nozzle section 14 is terminated. The supporting air stream is then stopped and the bed collapses on grid 18 and the coated tablets are removed from the apparatus in accordance with standard procedures in the art.

It will be apparent to those skilled in the art that various means can be employed to vary or control the velocity of the gas stream used to suspend the particles to be coated to obtain a cyclic flow pattern of the type described above. The grids described above, for example, can be replaced or be used in combination with conventional dampers or like movable plates for regulating the flow of gas. In the present invention this can be readily accomplished by positioning a standard damper in conduit section 16 to direct the higher velocity gas stream toward atomizer 30. Instead of retarding the flow of one part of a single gas stream, the gas supply system can also be made up of a multiple of gas streams of varying velocities to obtain the desired controlled cyclic flow of the particles through the coating zone. A preferred embodiment, however, includes the use of porous grids or screens as illustrated which can include the modification in which the portion of the screen directly above nozzle 30 is cut completely away and the nozzle positioned so that it is at or slightly above the level of the screen.

I claim:

The process of coating particles with coating material in a vertically disposed coating tower, whichcomprises forming an air suspended bed of particles in which all particles are suspended in an upwardly flowing air stream entering the bottom of said tower, said air stream entering the bottom of said tower being substantially coextensive with the cross-sectional area of said suspended bed in said tower, imparting controlled cyclic movement to 'the particles with a portion of the suspended bed flowing upwardly and the other portion of the suspended bed flowing downwardly without randomness of particle motion characteristic of fluidized beds, by directing the upwardly flowing air stream into the bottom of said tower in a high velocity upwardly flowing air stream in one part of said tower in which the particles are suspended and move upwardly concurrent to the upwardly flowing high velocity air stream substantially out of contact with each other in the portion of the suspended bed subjected to said high velocity air stream and pass to the top of the downwardly flowing portion of said bed, and a low velocity upwardly flowing air stream in the other portion of said tower in which the particles move downwardly and settle through the upwardly flowing low velocity air stream in substantially weightless contact with each other and in substantially undisturbed relation in the portion of the suspended bed subjected to said low velocity air stream and pass to the bottom of the'upwardly flowing portion of said bed, and directing all of said coating material into the high velocity air stream to provide for the coating of only the particles moving upwardly in said high velocity air stream.

References (Iited by the Examiner UNITED STATES PATENTS RICHARD D. NEVIUS, Primary Examiner.

JOSEPH B. SPENCER, Examiner.

United states-"Patent [191' Wurster [54] PARTICLE COATING PROCESS Dale E. Wurster, Madison, Wis.

Wisconsin Alumni Research Foundation, Madison, Wis.

[75] Inventor: [73] Assignee:

Reexamination Request No. 90/000,004, Jul. 1, 1981 [63] Continuation-impart of Ser. No. 861,063, Dec. 21,

1959, abandoned.

[51] Int. Cl. B05D 3/00; BOSD 1/72 [52] US. 427/213; 118/62; 118/303; 34/57A [56] References Cited U.S. PATENT DOCUMENTS 2,768,095 10/1956 Tadema 117/100 3,089,824 5/1963 Wurster... 3,196,827 7/ 1965 Wurster... 3,241,520 3/1966 Wurster...

FOREIGN PATENT DOCUMENTS 596,861 5/1961 Belgium. 656,429 l/ 1963 Canada. 676,215 12/1963 Canada.

OTHER PUBLICATIONS Hinkes, Tom, Seed Particle Coating, in Marshall, ed.. F luidized Bed Mixing, Granulating and Drying Technology, Section E [East Brunswick, New

[45] Certificate Issued Apr. 13, 1981 Jersey: The Institute rot Applied Pharmaceutical Sci ences (1980)] Primary Examiner-Shrive P. Beck EXEMPLARY CLAIM The process of coating particles with coating mate rial in a vertically disposed coating tower, whicl comprises forming an air suspended bed of particles i1 which all particles are suspended in an upwardl flowing air stream entering the bottom of said tower said air stream entering the bottom of said towe being substantially coextensive with the cross-section al area of said suspended bed in said tower, impartin; controlled cyclic movement to the particles with portion of the suspended bed flowing upwardly am the other portion of the suspended bed flowing down wardly without randomness of particle motion char acteristic of fluidized beds, by directing the upwardl; flowing air streaminto the bottom of said tower in high velocity upwardly flowing air stream in one par of said tower in which the particles are suspended ant move upwardly concurrent to the upwardly flowin; high velocity air stream substantially out of contac with each other in the portion of the suspended be subjected to said high velocity air stream and pass tt the top of the downwardly flowing portion of sait bed, and a low velocity upwardly flowing air stream in the other portion of said tower in which the parti cles move downwardly and settle through the up wardly flowing low velocity air stream in substantial ly weightless contact with each other and in substan tially undisturbed relation in the portion of the sus pended bed subjected to said low velocity air strear and pass to the bottom of the upwardly flowing poi tion of said bed, and directing all of said coatin material into the high velocity air stream to provid for the coating of only the particles moving upwardl in said high velocity air stream.

REEXAMINATION CERTIFICATE ISSUED UNDER 35 U.S.C. 307

NO AMENDMENTS HAVE BEEN MADE TO THE PATENT The patcmability of the claim is confirmed. 5 t 4: m =0: =1: 

